It was fairly easy after the penny had dropped and one sleepless night, even if I was away from home and my reference sources. But that Irishman ... It would have been tricky enough without the additional factors and a difficult enough clue.
I don’t have the puzzle to hand but from memory the last letter comes from a two letter interjection (I see!). I recall that google indicates the name is as much Scottish as Irish
Indeed ... the only famous person with that spelling appears to be one with the surname Erskine who was a strong advocate for Gaelic - but in Scotland ...